The present invention relates to multiplexing and demultiplexing optical signals. and in particular to a diffraction grating based wavelength multiplexer/demultiplexer.
Fiber optic telecommunications rely nowadays on a technique called xe2x80x9cDWDMxe2x80x9d (dense wavelength division multiplexing) to achieve higher bandwidth at lower cost. This technique utilizes many different wavelength bands each carrying an independent channel of information. These wavelengths usually span a 50 nm band in the 1.55 xcexcm region. The ITU has normalized these wavelengths to lie on a periodic frequency grid with a spacing of typically 100 to 200 GHz. The present invention relates to a wavelength demultiplexer. This is a special kind of optical filter having one input receiving the wavelength multiplexed channels and several outputs each receiving the separated wavelength channels. U.S. Pat. Nos. 4,111,524; 4,198,117; 4,522,462 and 4,763,969 disclose typical WDM devices. It should be noted that everything that is stated in this disclosure relates both to demultiplexers (1 input and N outputs) and multiplexers (N inputs and 1 output), which are the same devices with the inputs and the outputs reversed.
A typical realization of a diffraction grating based dense WDM wavelength demultiplexer is illustrated in FIG. 1. The typical diffraction grating demultiplexer includes: coupling optics 1, which can be as simple as the optical fibers themselves, or include an array of micro lenses (as in European Patent EP 0859249A1); collimating optics 2, which can be one or more lenses, or a curved mirror (on which the grating can be directly written if needed); and a diffraction grating 3.
The most compact configuration is called the Littrow configuration, where the reflected output beams from the grating are almost aligned with the input beam (like in FIG. 1).
In order to achieve sufficient wavelength separation in the focal plane, the device must have a high dispersion. This is either done using a large focal length lens or using a high groove frequency grating. However, diffraction gratings suffer from polarization sensitivity when the groove frequency is above ca. 600 lines/mm. This implies that the focal length of the lens should be at least 20 mm for 200 GHz spacing.
Another approach is to use higher groove frequency grating in conjunction with some polarization processing to maintain low polarization sensitivity. This is described for example in U.S. Pat. No. 4,741,588, issued May 3, 1988 to Antonius Nicia et al. and U.S. Pat. No. 5,886,785, issued Mar. 23, 1999 to Herve Lefevre et al. The polarization processing used in these patents is done when the beams are collimated (see FIG. 2).
The main purpose of the polarization diversity technique is to convert input light into two sub-beams with the same polarization state, the one that has the highest diffraction efficiency from the grating (usually at 1.55 xcexcm, it is a linear polarization perpendicular to the grating""s grooves). This is achieved through the combination of a polarization splitting device 4, usually splitting input light into two perpendicular linear states of polarization 6 and 7 parallel and perpendicular to the grating lines, and a waveplate 8. The polarization splitting device 4 is either a polarization beam splitter or a birefringent beam displacer. The waveplate 8 is usually a single half-wave plate oriented at 45xc2x0 with respect to the groove axis positioned in the path of one of the two sub-beams.
The main drawback of this technique is that since it is done with the collimated beams, which have relatively large diameters, this polarization-processing scheme is bulky.
An object of the present invention is to overcome the shortcomings of the prior art by providing a diffraction grating based WDM in which the polarization processing is executed prior to collimation.
Accordingly, the present invention relates to a wavelength division demultiplexer device comprising:
an input waveguide for launching an input multiplexed light beam into the device;
polarization beam splitting means for separating the input light beam into two polarized sub-beams;
first polarization rotating means for rotating one of the two sub-beams to provide two parallel polarized sub-beam, wherein a portion of one of the sub-beams is not properly polarized;
light collimating means for collimating the two parallel sub-beams;
diffraction grating means for separating the two sub-beam into a first and a second plurality of output light sub-beams; and
light blocking means in front of the diffraction grating for eliminating at least some of the portion of one of the sub-beams not properly polarized.